Flow equations in various cases
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©MBDCI 6-C Drilling Depleted Reservoirs Drilling Depleted Reservoirs Maurice Dusseault ©MBDCI Depleted Reservoir Effects Depletion of a zone has two major effects: lateral total stress, sh, drops The effective stresses, sh, sv rise The 6-C Drilling Depleted Reservoirs This results in: Drop in PF in the depleted zone Increase in confining stress = stronger rock Consequences: Slower drilling because rock is tougher LC and blowout risks go up substantially More casing strings, LCM squeezes, … More common recently (deeper targets) ©MBDCI Reservoir Depletion stress sh]o pressure depletion in stratum 6-C Drilling Depleted Reservoirs pf sh lateral stress redistribution po Dsh depleted sand Dp stress concentration depth sh]f Pressure depletion leads to an increase in sv, sh and a decrease in sh (Sh). This is the Poisson effect, related to n/(1-n) (n = symbol for Poisson’s Ratio) Increased s means the rock is stronger and tougher to drill. Different bits may be required. The sh “lost” in the reservoir zone must be redistributed above and below the reservoir ©MBDCI Depletion Effect on sh wellbore sh concentration far-field stresses 6-C Drilling Depleted Reservoirs sh stress trajectories final sh Z initial sh sh along wellbore Zone after production (Dp) Operational consequences: -low pfrac in reservoir -higher pfrac above reservoir -better fracture containment Depletion and sh (or PF) 6-C Drilling Depleted Reservoirs ©MBDCI The reservoir shrinks because of the drop in pore pressure sh“arches” around the reservoir In addition to a drop in PF, there is an increase in sh above and below the zone Effects on vertical stress are negligible: Because reservoirs are usually laterally extensive and thin in comparison to L PF can be expected to be low Most serious in HTHP wells, multiple zones ©MBDCI Shear, Fracture Opening sv Zone of pressure decline, -Dp 6-C Drilling Depleted Reservoirs sh Pressure decline leads to an increase in the shear stress This can lead to shearing, which causes fractures and fissures to open This leads to increases in permeability, better reservoir drainage Exceptionally, it can lead to casing problems, etc. ©MBDCI Other Depletion Effects 6-C Drilling Depleted Reservoirs In some rare cases, the reservoir enters a condition of shear failure (or collapse) This may have beneficial effects on the reservoir itself (e.g. Ekofisk in North Sea) Shearing in a dense rock opens fractures Permeability goes up Of course, there may also be issues of casing shear (Dusseault et al., 2001) Free gas development in the reservoir Seismic velocities change, stiffness too Dusseault MB, Bruno MS, Barrera J, 2001. Casing shear: causes, cases, cures. SPE Drilling & Completion Vol.16 Num. 2, 98-107, June 2001 … ©MBDCI Depletion Failure Path, M-C Plot t – shear stress 6-C Drilling Depleted Reservoirs Mohr-Coulomb plot before Dp, initial in situ conditions sh]b Y This example is for a normal fault case, assuming sv constant shear sh]a sv]b after Dp sv]a a & b stand for after and before Dp Remember, these are effective stresses, not total stresses sn ©MBDCI Irreconcilable MW in Depletion stress po sh]i 6-C Drilling Depleted Reservoirs undepleted sand MW to control po in undepleted sands depleted sand max. MW to avoid fracturing undepleted sand depth lateral stress depletion MW limits cannot be reconciled! Special measures are required. It is impossible to find a MW that both controls the pore pressure in thin adjacent undepleted sands, and also avoids LC in the depleted zone This effect is greatest in deep HPHT reservoirs that are massively depleted (large oil reservoir drawdown) ©MBDCI A Typical GoM Case 0 50 100 150 stress, pressure, MPa 1 pore pressure, po 6-C Drilling Depleted Reservoirs 2 thick shale sequence lateral stress, shmin vertical stress, sv 3 4 gas sand Target A gas sands Target B gas sand Target C 5 6 depth - kilometres A was discovered and depleted to 0.2po Now, we want to drill through to B & C Or, A & B depleted, and our target is C Small gas sands are not depleted and present blowout hazard Lowered PF in zones present LC hazard ©MBDCI Gradient Plot, GoM Case 1.0 2.0 0 16.7 ppg gradient plot, mud density 1 po shmin 6-C Drilling Depleted Reservoirs 2 thick shale sequence 3 4 5 sv po gas sand Target A gas sands Target B gas sand Target C depth - kilometres 6 Same as the previous plot, only now in mud density units Colored lines are the original state of po, s 2.0 = 16.7 ppg MW Original target(s) A&B are depleted The goal is to drill to Target C without: Too many casings Lost circ, blowout Challenging! ©MBDCI What Happens in the Reservoir 6-C Drilling Depleted Reservoirs Reservoir is thin, extensive, so sv constant No horizontal strains can take place Thus, the horizontal total stress changes are governed only by the Poisson’s ratio effect and the change in pore pressure σv σh σh σv po ex = ey = 0 ©MBDCI New Fracture Gradient Calculation 6-C Drilling Depleted Reservoirs Determine depleted pore pressure in ppg Determine pore pressure change in ppg New PF is fracture gradient is : PF = PF]i + Dp·(1-2n)/(1-n) Where PF]i is the initial fracture gradient Dp is the change in reservoir pressure in ppg (remember, this is a –ve number) n is Poisson’s ratio, a dimensionless value You can do these in any units (density, ppg, MPa, psi), as long as you are consistent ©MBDCI All Stresses After Depletion 6-C Drilling Depleted Reservoirs All the new stresses can be calculated New pore pressure is pf, change, Dp = po - pf Initial sv]i = sv – po, final is sv]f = sv – pf Horizontal effective stress change = Dsh = Dsv·n/(1-n) = -Dp·n/(1-n) (Dp is –ve) New horizontal effective stress = sh]f = sh]i - Dp·n/(1-n) (where Dp is –ve) PF = sh]f/z (density units where water = 1) Careful about the signs (+ve, -ve) of terms ©MBDCI Example of Depletion Calculation 6-C Drilling Depleted Reservoirs po = 14 ppg, pf = 4 ppg, Dp = -10 ppg PF]i = shmin = 16 ppg, assume n = 0.2 PF]f = PF]i + Dp·(1-2n)/(1-n) PF]f = 16 ppg +(-10·(1 - 20.2)/(1 - 0.2)) ppg = 16 ppg – 7.5 ppg = 8.5 ppg This is almost the gradient of H20! Lost circulation will be unavoidable! There will still be some thin 14 ppg gas sands above or below the zone! Let’s discuss choice of Poisson’s ratio ©MBDCI Choice of Poisson’s Ratio 6-C Drilling Depleted Reservoirs In very shaley sands, n ~ 0.28 – 0.32 In clean sands, n ~ 0.23 – 0.25 In fractured reservoirs, n ~ 0.18 – 0.22 Extremely fractured, as low as 0.12 - 0.15 You can use geophysical log estimates (~) Better, you can use calibrations based on real measurements, methods have been published It is best to calibrate to your own basin, so collect the data as it becomes available ©MBDCI Back to Our Example 1.0 2.0 0 16.7 ppg gradient plot, mud density 1 po 6-C Drilling Depleted Reservoirs 2 PF = shmin thick shale sequence 3 4 sv Now, there is no MW possible to drill through the zones without exceeding PF or below po We need to find a new strategy for drilling po Intact thin gas sands at original po Two depleted reservoirs, low PF 5 New target reservoir 6 Risks of blowouts, LC depth - kilometres ©MBDCI The Casings & Liners Option 6-C Drilling Depleted Reservoirs Requires expensive casing strings or liners This approach usually results in two extra strings for each depleted zone Clearly, it is prohibitively expensive if there is more than one zone with active charged gas sands between Are there other options? Yes, but there are some risks involved, or, there is additional time involved Let’s look at some options ©MBDCI Option One: Use Casings/Liners 1.0 2.0 0 16.7 ppg gradient plot, mud density 1 PF 3 13.0 ppg 6-C Drilling Depleted Reservoirs 2 4 A B C D 5 6 depth - kilometres Drill to below A with 16 ppg MW, set casing Lower MW to 13 ppg, drill to B, set casing Raise MW to 16.7, drill to C, set casing Lower MW to 13.5 ppg, drill to D, set casing Raise MW to 16.7 ppg, drill to TD, set production casing/liner Now you have only a 5” hole and a 3.5” “casing” ©MBDCI Case with no Overlying Gas Sands This is a favorable condition: Drill to bottom of depleted zone w. low MW LCM squeeze in depleted zone to raise PF Raise MW to drill ahead, resqueeze if needed 6-C Drilling Depleted Reservoirs LCM squeezing is performed as much as necessary to raise MW to po in gas sands However, remember the shales! po is still high in the shales If MW < po(shale), massive sloughing possible This option is similar to underbalanced drilling Thin undetected gas stringers remain a risk! ©MBDCI No Overlying Gas Sands Perhaps UB drilling will help in this zone 6-C Drilling Depleted Reservoirs Depleted Gas sands Target Drill down to top of zone with as low MW as possible Add LCM to mud before you enter zone Build mud & LCM to keep up with losses Set casing below zone Increase MW to handle po in gas sands Drill to Target ©MBDCI Depleting the Thin Gas Sands 6-C Drilling Depleted Reservoirs If the original wells are still available… Perforate into all the thin gas sands identified on logs or drilling records Deplete with maximum drawdown to reduce PF to the same as adjacent depleted zones Drill with low MW, set casing above target, increase MW, drill into target… Risks? Some thin sands may be missed Depletion may be a lengthy process Drill immediately after depletion and set casing as quickly as possible (low recharge time) ©MBDCI Living with Some Gas Influx 6-C Drilling Depleted Reservoirs If the gas sands are tight and thin… Gas influx rate may be manageable during drilling with the MW less than the pressure in the gas sands (underbalanced) Also, shales cannot be so weak that they slough into the hole with the low MW used! Mud de-gassers, gas collection from closed flow lines, etc. are safety measures This can be used only with great caution in the right circumstances Unlikely in sloughing shale regions ©MBDCI Is it Possible to Avoid the Zones? 6-C Drilling Depleted Reservoirs If the depleted zones are in a fault block, or are limited channel sands, avoidance… Using seismics, drill into zone to avoid the other zones Good stratigraphic/seismic data are vital Well trajectory for avoiding the depleted sequence Gas sands Depleted Target Faults isolating the productive sequence ©MBDCI Can Old Wells be Re-Used? If the old wells are in good condition, they can be plugged carefully and used to drill 6-C Drilling Depleted Reservoirs Squeeze with non-shrinking cement, perhaps patches too Repeat squeeze perhaps? Drill ahead (with bicenter bit?) Install liner (expandable liner to save hole size?) Complete the well Multilaterals are also possible… ©MBDCI LCM Squeezes in Depleted Zones 6-C Drilling Depleted Reservoirs Drill to just above the depleted zone with as low a MW as feasible Place high LCM mud at bit, and drill into zone OB; LC will occur, but will also tend to “pack off” the depleted zone Drill cautiously through zone, continuing to allow fracturing with the high LCM mud Once through zone, it may be advisable to close annulus and slowly squeeze extra LCM into the depleted zone Measure LOT in depleted zone for safety ©MBDCI How Does a LCM Squeeze Work? Zone of increased sq Short, wide fractures created 6-C Drilling Depleted Reservoirs This increases the stresses locally around the wellbore hole Multiple fractures shmin sHMAX sq must go up as material packed in Effect can be increased by repeated squeezes to make the tangential stresses higher in a larger zone around well However, remember that does not increase stresses beyond the extent of the fractures. If a high po is encountered lower, LC may take place ©MBDCI Some Rig Capabilities Needed 6-C Drilling Depleted Reservoirs For drilling HPHT wells, and for drilling deep through depleted zones High & variable load capacity for riser and mud storage on offshore rigs High pump capability Large diameter kill line and choke line needed for quick response Early kick detection system Pressure sensors on BOP at mud-line level Gas handler on marine riser Pressure measurement devices on MWD LCM pill mixing facility for LCM squeezes ©MBDCI Depleted Zone Options Summary Use the old wells if possible Deepen wells after zonal isolation Can be used to deplete charged thin sands 6-C Drilling Depleted Reservoirs Can the new well trajectory avoid zones? Live with gas cut mud (if shales are strong) Drill with designed LCM material in mud Use LCM squeeze in depleted zones Place strategic casings and liners Use bicentered bits, expandable casings to maintain reasonable hole diameter with depth Combine one or more of these options ©MBDCI Lessons Learned- Depleted Cases 6-C Drilling Depleted Reservoirs Depleted reservoirs present singular challenges for drilling Options include avoidance, use of LCM, multiple casings/liners, use of old wells… The risks involved are higher than in conventional reservoir development Careful surveillance during drilling Special drilling equipment… Bicenter bits and expandable liners represent a useful, albeit costly approach
